systemctl start mastodon-streaming

gistfile1.txt

Jun 15 14:05:59 archlinux.elou.world systemd[1]: Started mastodon-streaming.
Jun 15 14:06:00 archlinux.elou.world npm[18372]: > mastodon@ start /var/lib/mastodon/live
Jun 15 14:06:00 archlinux.elou.world npm[18372]: > rimraf ./tmp/streaming && babel ./streaming/index.js --out-dir ./tmp && node ./tmp/streaming/index.js
Jun 15 14:06:02 archlinux.elou.world npm[18372]: ./streaming/index.js -> tmp/streaming/index.js
Jun 15 14:06:03 archlinux.elou.world npm[18372]: /var/lib/mastodon/live/node_modules/uws/uws.js:38
Jun 15 14:06:03 archlinux.elou.world npm[18372]:             throw new Error('Compilation of µWebSockets has failed and there is no pre-compiled binary ' +
Jun 15 14:06:03 archlinux.elou.world npm[18372]:             ^
Jun 15 14:06:03 archlinux.elou.world npm[18372]: Error: Compilation of µWebSockets has failed and there is no pre-compiled binary available for your system. Please install a supported C++11 compiler and reinstall the module 'uws'.
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at native (/var/lib/mastodon/live/node_modules/uws/uws.js:38:19)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Object.<anonymous> (/var/lib/mastodon/live/node_modules/uws/uws.js:42:3)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Module._compile (module.js:569:30)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Object.Module._extensions..js (module.js:580:10)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Module.load (module.js:503:32)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at tryModuleLoad (module.js:466:12)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Function.Module._load (module.js:458:3)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Module.require (module.js:513:17)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at require (internal/module.js:11:18)
Jun 15 14:06:03 archlinux.elou.world npm[18372]:     at Object.<anonymous> (/var/lib/mastodon/live/tmp/streaming/index.js:39:12)
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! code ELIFECYCLE
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! errno 1
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! mastodon@ start: `rimraf ./tmp/streaming && babel ./streaming/index.js --out-dir ./tmp && node ./tmp/streaming/index.js`
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! Exit status 1
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR!
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! Failed at the mastodon@ start script.
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR! A complete log of this run can be found in:
Jun 15 14:06:03 archlinux.elou.world npm[18372]: npm ERR!     /var/lib/mastodon/.npm/_logs/2017-06-15T12_06_03_519Z-debug.log
Jun 15 14:06:03 archlinux.elou.world systemd[1]: mastodon-streaming.service: Main process exited, code=exited, status=1/FAILURE
Jun 15 14:06:03 archlinux.elou.world systemd[1]: mastodon-streaming.service: Unit entered failed state.
Jun 15 14:06:03 archlinux.elou.world systemd[1]: mastodon-streaming.service: Failed with result 'exit-code'.

I just saw the response lol. In case someone still gets the issue, I'm going to reply.
You need to find the repository (it regularly changes place, thanks to the maintainer mind) that can be used to build the native code to be used by uws, then copy the built .node file into node_modules/uws directory. You may want to edit the Makefile to only build your version (or adapt if it cannot build yours).
Right now you can find the repository here.
Note that a fork exists, cWS, if something goes wrong again with uws.